Whilst the following discussion concerns locking differentials for vehicles it is to be understood that the same principles apply to machines which have the need of a differential.
Basically a differential is incorporated in an automobile in association with the back wheel axles. This provides a means by which each wheel can be rotated at different relative speeds. The need for a difference in the respective speeds of wheels will be readily apparent when considering this in the context of cornering of a vehicle.
In this respect the conventional differential includes a pinion gear, a ring gear, differential side gears and differential pinion gears. The working relationship of these elements is well known.
It is common for such type of differentials to permit one wheel to rotate independently of an opposite wheel, for example, a pair of back wheels in a rear wheel drive vehicle. In particular, if the wheels of the vehicle encounter slippery surfaces such as mud or ice, skidding may occur. In this instance the wheel encountering the slippery surface will spin and the rotation of the opposite wheel is correspondingly impeded. This can result in the spinning wheel becoming bogged with little opportunity of the rotation of the opposite wheel on firm ground driving the vehicle away from the slippery surface. In such situations it is desirable to restrain the rotation of one wheel so it may be generally synchronous with the rotation of the opposite wheel. This is known as the lockable differential which ensures that the two wheels rotate at essentially the same rate. In this way the wheel which has a firm contact with the ground compensates for the other wheel that is on a slippery surface.
In order to gain the advantages of the conventional differential and the lockable differential, a compromise was designed and known as the limited slip differential. This type of differential allows some speed variations between the wheels but gives sufficient synchronisation between the pair of opposing wheels to be effective on slippery surfaces.
The present invention relates to the lockable type of differential. One example of a lockable differential is disclosed in Australian patent no. 534218. This differential relies upon the use of coaxial axles, one of which can be moved into the internal cavity of the differential and engage the carrier. This locks the differential. Dog clutches have also been used previously in a similar fashion so that the coaxial axles actually inter-engage and lock the differential.
Another example of a known lockable differential is disclosed in Australian patent no. 565257 which proposes the use of an annular piston. By moving the piston, a sleeve is moved which causes an annulus to engage a bevel gear and lock the differential. In use, this type of differential locking system is actuated by applying compressed air to one side of the piston. However, this annular piston is a complex design which is expensive to manufacture and time consuming to install.